The invention relates to a magnetic-inductive flow meter with a measuring tube made of metal, which is flowed through by a medium which has a minimum electrical conductivity and is permeated by a magnetic field generated by means of an arrangement of magnets, in the region of which field at least two measuring electrodes arranged diametrically on the measuring tube are provided for sensing the inductively generated measuring voltage, the metal measuring tube being provided with an electrically insulating inner layer and an electrically conductive grounding disk being arranged at the end face for grounding the medium flowing through the measuring tube, which disk is connected to ground via the measuring tube.
The area of use of such a magnetic-inductive flow meter extends to the determination of volumetric or mass flows of a flowable medium, such as liquids, slurries, pastes and the like. The flowable medium must in this case have a specific minimum electrical conductivity in order for the measuring method to work. Flow meters of the type of interest here are distinguished by quite accurate measuring results, without any pressure loss being caused in the pipeline system by the measurement.
Furthermore, magnetic-inductive flow meters do not have any movable components or components protruding into the measuring tube, which are particularly liable to wear. The flow meters of interest here are used primarily in the chemical industry, pharmaceuticals and the cosmetics industry as well as communal water and waste-water management and the food industry.
Faraday's law of induction forms the basis for the measuring method. This natural law states that a voltage is induced in a conductor moving in a magnetic field. When this natural law is exploited in measuring technology, the electrically conductive medium flows through a measuring tube in which a magnetic field is generated perpendicularly in relation to the direction of flow. The voltage induced in the medium as a result is picked up by an arrangement of electrodes. Usually two measuring electrodes inserted opposite each other in the measuring tube are used as the arrangement of electrodes. Since the measuring voltage obtained in this way is proportional to the average flow rate of the flowing medium, the volumetric flow of the medium can be determined from this. Taking the density of the flowing medium into account, its mass flow can be ascertained.
It is generally known that the measuring tube of a magnetic-inductive flow meter can be produced completely from plastic. Since the plastics used are electrical insulators, no separate electrical insulating measures have to be taken for the arrangement of the measuring electrodes in the wall of the measuring tube and the like. However, measuring tubes of plastic are generally only pressure-resistant to a limited extent.
DE 102 60 561 A1 discloses a flow meter which has a measuring tube made of metal, with the result that the problems indicated above do not exist in the case of this construction. Since, however, a metal measuring tube is electrically conductive, local insulating measures are required. For example, the measuring electrodes are inserted over an insulating layer with respect to the measuring tube. In particular, the inner surface of the measuring tube is lined or coated with an electrically nonconductive material. Polytetrafluoroethylene (PTFE) is suitable for this for example. The electrically insulating lining of the metal measuring tube is required in order for the measuring principle, which is based on an induction of voltage into the flowing medium, to work. The electrically insulating inner layer of a metal measuring tube may be produced in various ways. On the one hand, it is known to draw a plastic tube—known as a liner—into the metal measuring tube and fixed it there with respect to the measuring tube. On the other hand, an electrically insulating plastic may be sprayed onto the inner surface of the metal measuring tube. After curing, the electrically insulating inner layer is obtained.
For the measuring principle used here to work, it is additionally also required to ground the medium which flows through the measuring tube. Additional grounding means which are connected to ground via the in any case grounded metal measuring tube and transfer the ground potential to the medium flowing through are customary here for ground potential application. For this purpose, additional grounding electrodes may be used, inserted just like the measuring electrodes into the wall of the metal measuring tube. However, this is quite laborious in production engineering terms. Therefore, a grounding disk is usually used for the purpose of ground potential application of the medium flowing through. The grounding disk is annular and is flowed through by the medium. The grounding disk is arranged on the side of the flange of a measuring tube that is on the inlet side, alongside a sealing ring sealing the flange of the measuring tube with respect to the grounding disk. The grounding disk placed in this way is connected to the grounded metal tube via a grounding cable. The connection is formed by means of a soldered and/or screwed connection. This has the result that there is the risk of the exposed grounding cable being torn away. In addition, the fitting and electrical connection of the conventional grounding disk is quite laborious.